1. Field of the Invention
This invention relates to the field of video data compression. More particularly, this invention relates to the field of video data compression in which frequency separation and quantization are used.
2. Description of the Prior Art
British Published Patent Application GB-A-2 251 756 (Sony Broadcast & Communications Limited) describes a system using sub band frequency separation and quantization as part of a data compression process. Sub band frequency separation is used to decorrelate the video data into spatial frequency components of uniform size within the two-dimensional spatial frequency domain. The spatial frequency component data is then subject to a quantization process whereby the data representing the spatial frequencies to which the human visual system is less receptive is quantized more heavily than other spatial frequency component data. The degree of quantization applied to each spatial frequency component is chosen to follow an empirically determined relationship between relative human visual responsiveness at differing spatial frequencies. In general, the lower spatial frequency components are less heavily quantized than the higher spatial frequency components.
Another video data compression system is described in IEEE Transactions on Circuits and Systems for Video Technology, Volume 1, No. 2, June 1991, pages 174 to 183, "Sub band Coding Algorithms for Video Application: Videophone to HDTV-Conferencing"by Hamid Gharavi. In contrast to the above, a video compression system is disclosed which separates the data into sub band components in the two-dimensional spatial frequency domain of non-uniform size. This form of frequency separation is also known as "wavelet" coding. The frequency separated data is passed via a quantizer to a buffer store. The buffer store is continuously loaded with the compressed data and continuously emptied as the data is read for transmission. The degree of quantization is controlled based upon the spare capacity within the buffer store so as to avoid a disadvantageous overflow or underflow of the buffer store.
A constant aim within video data compression systems is to achieve a higher degree of data compression with a reduced degree of degradation in image quality upon decompression. Generally speaking, the higher the degree of compression used, the greater will be the degradation in the image when it is decompressed. More specifically, higher degrees of compression can be achieved by using heavier quantization (i.e. larger quantization step widths), but quantization is a non-reversible process and heavier quantization will result in an increase in degradation in the image when it is decompressed.